Carbon monoxide generated by incomplete combustion of carbon when using gas appliances or kerosene appliances, or when smoking, is a very highly toxic substance to the human body, as well-known.
There is known a conventional method for removing carbon monoxide in which carbon monoxide is adsorbed onto adsorbents such as zeolite, activated carbon and silica, and removed. A method is also known, as described in Japanese Unexamined Patent Publication No. 5-111618, in which a composite metal oxide (hopcalite) composed of manganese oxide, copper oxide, cobalt oxide and silver oxide is used as a catalyst. A further method is known in which carbon monoxide is adsorbed onto a platinum-group noble metal catalyst such as platinum, iridium, osmium, palladium, rhodium, or ruthenium, and decomposed.
Recently, some catalysts are proposed for removing carbon monoxide in an atmospheric environment. An example of the catalysts is a carbon monoxide decomposition catalyst obtained by heating of proteins containing metals such as iron and copper, as described in Japanese Unexamined Patent Publication No. 2006-167645. A further example is a carbon monoxide photo-decomposition catalyst comprising titanium oxide having cover of tungsten oxide thereon, as described in Japanese Unexamined Patent Publication No. 2007-500077.
Meanwhile, in industrial applications, for example, hydrogen fuel which is obtained from a reformed gas and used for polymer electrolyte fuel cells is required to reduce the concentration of carbon monoxide therein to a level of several to several tens of ppm in order to prevent poisoning of platinum used as an electrode catalyst in the fuel cell. Therefore, hydrocarbons such as natural gas, naphtha and kerosene, or alcohols such as methanol are steam-reformed to manufacture a reformed gas containing hydrogen as a main component, the reformed gas is subjected to carbon monoxide conversion, and then carbon monoxide is oxidized and removed, thereby hydrogen suitably used for fuel is manufactured.
Here, as a catalyst for oxidation of carbon monoxide, for example, ruthenium is known, as described in Japanese Unexamined Patent Publication No. 2002-356310, and adsorbents such as platinum, palladium, or the like are also known for selective adsorption of carbon monoxide contained in a reformed hydrogen gas, as described in Japanese Unexamined Patent Publication No. 9-10538. However, the catalysts and adsorbents including these noble metals are expensive as well as the oxidizing agent for carbon monoxide may react with hydrogen as a main component in a reformed gas to reduce the hydrogen concentration therein.
Meanwhile, crystalline zeolite and activated alumina are conventionally well-known as adsorbents. Among the crystalline zeolite, MS-4A which is industrially in wide use has pores with an average particle diameter of about 0.4 nm, and MS-13X has a uniform pore diameter with an average particle diameter of about 1.0 nm. In contrast, porous activated alumina has pores with an average pore diameter of about 10 nm, a specific surface area of 50 to 400 m2/g and a pore volume of 0.1 to 1.0 cm3/g.
In recent years, a mesoporous silica which is crystalline porous silica having mesopores with a larger average pore diameter than that of such zeolites, namely, with an average pore diameter of 2 to 50 nm has been synthesized. Noting that the mesoporous silica has such larger mesopores than those of the zeolite, an application as a catalyst support for various catalytic reactions is proposed, as described in, for example, Japanese Unexamined Patent Publication No. 2005-238060. In addition, very recently, an application as a photometathesis catalyst of mesoporous silica by itself is proposed, as described in Phys. Chem. Chem. Phys., 2000, 2, 5293.
Silica gel is also well-known as an adsorbent having pores with an average pore diameter of 1 to 100 nm as well as having a large pore volume. Moreover, because of its chemical stability, silica gel is widely used in various industrial fields, for example, as a moisture-proof material for foods or pharmaceutical products, a dehydrating agent and a purification agent for gas or liquid, as well as a catalyst support.
As described above, the silica gel itself has been widely believed to be chemically inactive, but it is recently reported in J. Chem. Soc., Faraday Trans., 90 (14) 1994, 2107-2111, that amorphous silica having a high specific surface area (500 to 600 m2/g) absorbs ultraviolet ray (240 to 265 nm) and gives a broad emission spectrum around 440 nm, and hence it has photocatalytic function. In fact, photometathesis reactions by using amorphous silica are reported, and at the same time, comparing and discussing other adsorbents such as silica-alumina and alumina, it has been found that they give only a small amount of products from other than the metathesis reaction. Thus, as described in J. Chem. Soc., Chem. Commun., 1995, 761, discussion has been made about differences of such other adsorbents from the amorphous silica which typically gives an olefin metathesis reaction.
The present inventors, in the study for obtaining novel and useful methods for removing carbon monoxide present in oxygen-containing air or a specific gas atmosphere such as a gas atmosphere containing hydrogen or nitrogen as a main component, have carried out intensive studies focusing on the adsorptivity and photocatalytic function of porous silica as described above, and as a result, have found that carbon monoxide present at a low concentration in an oxygen-containing gas phase can be easily and effectively photooxidized to carbon dioxide by using these characteristics of porous silica, and the present invention has been accomplished.
Accordingly, it is an object of the invention to provide a method for photooxidizing carbon monoxide present at a low concentration in an oxygen-containing gas phase to carbon dioxide by using the adsorptivity and photocatalytic function of porous silica.